1. Field of the Invention
The present invention relates to a method of tracking satellite signals for Global Positioning System (GPS) receivers.
2. Description of the Invention
Positioning system currently in service utilizing artificial satellite include a so-called Global Positioning System (GPS). This positioning system, as the name implies, will cover the entire surface of the Globe by a total of 24 geodetic satellites when all of them are operational on six orbits, approximately 20,200 km above the Earth, wherein four geodetic satellites are assigned for each orbit. Each geodetic satellite transmits a GPS signal containing navigational data for reckoning to the Earth using a spread-spectrum system. The reckoning is carried out on the ground, on the sea as well as in the air by receiving GPS signals from a plurality of geodetic satellites, for example, by receiving GPS signals from three geodetic satellites for two-dimensional positioning and that from four geodetic satellites for three-dimensional positioning. In this way, based on the navigational data contained in the GPS signal from each geodetic satellite, position information at the receiving point such as a latitude, longitude and altitude thereof can be reckoned in real time.
This GPS system was originally developed for U.S. military use, however a part of the GPS signal (C/A code) has been made available for civil use. Therefore, it is possible to build navigation systems for automobiles, ships and aircraft by using the GPS signal.
A GPS receiver used for a vehicle-mounted navigation equipment starts a search operation to recapture the GPS satellite when the vehicle enters a tunnel, for example, and the receiption of the GPS signals from the GPS satellites is interrupted for more than a specified period of time (for example, one minute). A schematic diagram for such process is shown in FIG. 1.
The GPS satellites transmsit signals on a frequency of 1575.42 MHz, however, since they are not geostationary, the receiving frequency may shift due to the Doppler effect. The Doppler frequency shift caused by the orbiting GPS satellite is about .+-.5 KHz on the ground.
The movement of the GPS receiver itself or, in case of a vehicle-mounted GPS receiver, the movement of the vehicle may also cause the Doppler effect. The maximum Doppler frequency shift may become .+-.600 Hz when the vehicle moves at a speed of 60 m/sec. There is also a frequency shift of, approximately, .+-.3 KHz in a oscillator of the GPS receiver. When these frequency shifts are added, the resultant total frequency shift will be 5,000+600+3,000=8,600 Hz. This means that the frequency of the received satellite signal may shift to the maximum extent of .+-.8,600 Hz.
Accordingly, if an attempt to recapture the lost GPS satellite is made by the GPS receiver, it is necessary for the GPS receiver to search after the satellite signal within the maximum frequency shift range of .+-.8,600 Hz, with the frequency of 1575.42 MHz in the center, by changing the search frequency widely.
Generally, the GPS receiver adopts a phase-locked loop (PLL) circuit in the receiving circuit in order to tune the receiving frequency of the GPS receiver accurately in the GPS satellite frequency. There is no problem if a capture range of the PLL circuit is set widely enough in covering the maximum frequency shift range of .+-.8,600 Hz. However, in practice, due to structural limitations of the circuit configuration, the capture range of the PLL circuit is forced to be as narrow as .+-.150 Hz=300 Hz.
Therefore, in order to capture the satellite signals by searching through the maximum frequency shift range of .+-.8,600 Hz in the receiving frequency, it is necessary to divide the search frequency of the PLL circuit into several capture ranges and perform several searching steps for covering the overall frequency range. It is assumed, for example, that one PLL capture range has an extent of .+-.150 Hz=300 Hz, a number of steps to be performed in searching is 8,600/300=28.66. That is, the search frequency should be switched as many as 28 steps for one side of the transmission frequency of the GPS satellite, and the total of 56 steps are required to cover the overall search range for the both sides.
As mentioned previously, since the satellite signal is transmitted in accordance with the spread-spectrum system, the received signal should be despread firstly to identify whether or not the signal received is the one transmitted from the target GPS satellite. This identifying takes about 1 second, accordingly, 56 steps of searching operation require at least 56 seconds and the essential reckoning for positioning cannot be performed within this period of time.
Furthermore, the GPS receiver needs to capture and track at least three, desirably four, GPS satellites for positioning. The most preferable configuration of the GPS receiver is to allot one receiving channel to each GPS satellite. However, this is not practical for public instruments such as a vehicle-mounted navigational equipment in terms of requirements such that accuracy in positioning, low production cost, miniaturization and the like. Therefore, a single receiving channel is employed normally in the vehicle-mounted navigational equipment, and thereby the single receiving channel is multiplexed by time-division to enable the GPS receiver to receive satellite signals from the plurality of GPS satellites in sequence. In case of recapturing, for example, N satellites for tracking by using a sequential receiving type GPS receiver, at least 56.times.N seconds are necessary for in making a round of the recapturing operation for all the N GPS satellites, as shown in FIG. 2.
When, for example,s the PLL search frequency is in tune to the receiving frequency of the GPS satellite, and the signal from the GPS satellite is happened to be interrupted by buildings when the vehicle comes to a halt at stop signal, it may take another 56.times.N seconds for the search frequency to tune in again to the GPS satellite frequency. There has been a problem for the prior art GPS receiver, in this way, as it takes a considerable time in recapturing the lost GPS satellites and restarts the reckoning for positioning with excessive delay.
It is therefore an object of this invention to eliminate the problem encountered in the prior art GPS receiver and to provide a satellite signal tracking method for a GPS receiver through which lost GPS satellite can be recaptured within the shortest possible time.